This section provides background information related to the present disclosure which is not necessarily prior art. This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
Due to the increased potential applications in the areas of wireless communication systems, imaging systems, atmospheric studies, autonomous vehicle control, perimeter security, and the like, millimeter wave (MMW) range received extensive attention over the past decades. In this region, the wavelength is short enough to allow fabrication of compact size radars compatible with Monolithic Microwave Integrated Circuit (MMIC) chips and achieve higher resolution. Yet, at the same time, the wavelength is long enough at the lower band to allow signal penetration through environment with low visibility, such as smoke or fog, with little or no attenuation. MMW radar is also able to function in adverse weather conditions compared to optical sensors, such as lasers. On the other hand, since the small atmospheric particles, such as raindrops, can no longer be considered small compared to the wavelength at higher MMW bands, MMW radars have been extensively used for the remote sensing of clouds, snow covered vegetation, and the like.
Although the atmospheric absorption increases at higher frequencies, current activities in MMW region have focused on measuring across extremely short distances below 100 meters or so and therefore, in most cases, have been able to exclude any serious absorption on backscattering effects. In addition, the available bandwidth at each principal window of MMW band is extremely large, resulting in many advantages such as higher data rate and range resolution.
Recent demands for very high resolution radars highlighted the need for developing new methods for low-cost MMW radars. It is desirable to devise a means of providing electronic, rather than mechanical, beam scanning in order to reduce system complexity and cost. It is especially important to eliminate the use of gimbals because they are slow, bulky and susceptible to mechanical failure and because they experience strong mechanical forces that sharply limit the scanning speed. On the other hand, electronic beam steering radars are fast but rather expensive and power inefficient, requiring several Watts of power. In addition, the incorporated phase shifters are bulky and in most cases not available at higher MMW band.
Considering these limitations, a traveling-wave frequency scanning approach is the simplest method of beam steering if enough bandwidth is available for the radar operation. In a traveling-wave frequency scanning antenna array, scanning is achieved as a result of the frequency dependence of the complex propagation constant of the wave propagating inside the waveguide. Principally, elements are fed in series with a transmission line having appropriate delay line segments between two adjacent elements. The delay lines are equal in length and provide the progressive phase difference among the array elements. As the frequency is swept, the delay lines provide different values for the phase difference and cause beam steering. At the center frequency, delays are designed to keep all elements in phase, and the radiation is in the broadside direction. Taking advantage of transmission lines to generate the desired phase shift eliminates the need to use electronic phase shifters which require additional power to operate, and reduces the cost of the device. Moreover, the problem of connecting the miniature MMIC chip to the external antenna is solved because the phase shifters and radiating elements are now in one unit and can be fabricated on a single substrate.
Travelling-wave antennas are designed based on either dielectric materials which result in slow wave radiation or hollow structures which result in fast wave radiation. In upper MMW spectrum, excessive conductor loss in the complex feeding networks is a major problem. In addition, printed transmission lines, such as microstrip, require very thin substrates to avoid exciting surface waves. Construction of scanning arrays based on hollow waveguide structures proves to be convenient because it provides enough bandwidth, does not incorporate dielectric materials, yet presents high power handling capabilities and lower loss, especially at higher frequencies, compared to planar transmission lines. In these travelling-wave structures, the length of the waveguide provides the desired phase shift, while the radiation is through slots cut on the walls of the waveguide making it a leaky wave structure. Another advantage of the hollow waveguides is they are light weight, which makes them attractive when a large structure, like an array, is required. This feature especially finds applications in Micro Autonomous Systems and Technology (MAST) when the antenna should be mounted on a mobile platform. Moreover, at higher frequencies, as the dimensions of the lines and waveguides shrink, micromachining offers easy fabrication of complex structures with low cost and low mass.
There have been several attempts to fabricate W-band waveguides with low-cost microfabrication techniques, such as lithography. However, in these techniques, the height of the waveguide is limited by the maximum thickness of the spun photoresist, limiting the fabrication to the reduced-height waveguides which suffer from high attenuation. Taking advantage of the “snap-together” technique, a rectangular waveguide was fabricated in two halves and then the halves were put together to form a complete waveguide. An alternate technique to etch the waveguide is deep reactive ion etching (DRIE) of silicon. Unlike wet etching, which is dependent on the crystal planes of silicon, DRIE is anisotropic and provides vertical sidewalls. Hence, DRIE is a viable approach for fabrication of high-performance micromachined waveguide structure. In some cases, a feed transition using microfabrication processes with separately fabricated and assembled probes has been reported for both diamond and rectangular waveguide. Another high-precision silicon micromachined transition with a capability to integrate filters has been proposed and shows wideband characteristics at the same frequency range. A very simple transition from cavity-backed co-planar waveguide (CBCPW) to rectangular waveguide for micromachining applications has been proposed and tested in Ka-band.
According to the principles of the present teachings, a two-dimensional micromachined meander-line frequency scanning array using WR-3 rectangular waveguide is presented for Y-band applications. This structure is capable of achieving ±25° scanning around the broadside angle. A narrow 2° beamwidth is achieved in the azimuth direction using linear array of slots cut on the broad wall of the waveguide. Employing hybrid-coupled patch arrays, a fixed beam can be realized to present a fairly narrow beamwidth in the elevation direction as well. The waveguide is fed through a membrane-supported cavity-backed co-planar waveguide (CPW), which is the output of a frequency multiplier providing 230˜245 GHz FMCW signal.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
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